1. Field of the Invention
The present invention relates to a thin film transistor, a thin film transistor substrate and a simplified method of manufacturing the same, which provides a liquid crystal display device having high resolution.
2. Description of the Related Art
A liquid crystal display (“LCD”) device displays an image by adjusting optical transmittance of liquid crystals having dielectric anisotropy by using an electric field. The LCD device includes an LCD panel having the liquid crystals aligned in a form of an active matrix and a driving circuit for driving the LCD panel.
The LCD panel includes a color filter substrate and a thin film transistor substrate bonded to the color filter substrate by a sealant with the liquid crystals disposed therebetween.
The color filter substrate typically includes a black matrix, a color filter and a common electrode, stacked on an insulating substrate.
The thin film transistor substrate includes gate and data lines provided to intersect each other on a lower insulating substrate and a thin film transistor (“TFT”) connecting a pixel electrode and the gate and data lines. The TFT supplies a data signal from the data line to the pixel electrode in response to a scanning signal from the gate line. TFTs typically use amorphous silicon, polysilicon or a zinc oxide based substance as an active layer. The TFT using amorphous silicon can be manufactured at a low temperature, which has low mobility and fails to meet a condition of a constant current. The TFT using the polysilicon has high mobility and meets the condition of the constant current. However, the TFT using the polysilicon has difficulty in achieving a large scale due to its poor uniformity and needs a high temperature process. On the other hand, the TFT which includes a zinc oxide based semiconductor layer can be manufactured by a low temperature process and has appropriate mobility, satisfying the condition of the constant current.
However, the TFT including the oxide semiconductor layer employs transparent electrodes of ITO (indium tin oxide), Au or the like for source/drain electrodes. When the source/drain electrodes are formed of the transparent electrodes and a large area is covered, the resistance rises to bring about a considerable signal delay.
Moreover, when the source/drain electrodes are formed of Au, the material cost is increased.